In many applications, it is desired to know in real time and with an optimum quality at least one rotation parameter of a rotary member, such as the position, speed, acceleration or direction of movement thereof.
To do this, document WO-2006/064169 proposes the use of an encoder intended to be rigidly connected to the movable member and on which is formed a magnetic track that is able to emit a pseudo-sinusoidal magnetic field at a reading distance of a sensor comprising a plurality of sensitive elements.
Advantageously, each sensitive element may comprise at least one pattern based on a tunnel magneto resistance (TMR) material the resistance of which varies according to the detected magnetic field, such as, for example, described in document WO-2004/083881.
In order to determine a movement parameter of the movable member according to the evolution of the detected magnetic field, document WO-2006/064169 provides a combination of signals representative of the resistance of each of the sensitive elements in order to deliver two signals in quadrature and of the same amplitude that can be used to calculate the parameter. Furthermore, this solution provides a two-by-two subtraction of four signals, in order to obtain two signals in quadrature that are free of common noise.
In particular, the encoder comprises an alternating succession of North and South poles defining a constant polar width
      L    p    =            π      ⁢                          ⁢      R        Npp  along the reading radius R for a given number Npp of pairs of poles, the sensitive elements being equally distributed from a distance
  Lp  2to be able to deliver the signals in quadrature.
In some applications, the encoder must have a small number of pairs of poles, typically less than 6, so that the polar width Lp thereof becomes wide, in particular of the order of ten millimetres.
However, these wide poles deliver a magnetic signal the sinusoidality of which is poor at low reading air gap, requiring a distancing of the sensitive elements from the magnetic track, which goes against the amplitude of the signal and therefore the good detection thereof by the sensitive elements.
In addition, wide poles require a thickness of the encoder which is also greater in order to preserve the sinusoidality and the amplitude of the magnetic signal. This is not favourable to the integration of the encoder in reduced dimensions and complicates the magnetisation method because a greater thickness must be magnetically saturated.
Moreover, it is known, in particular from document DE-103 09 027, encoders the magnetic transitions of which between the North and South poles extend along an Archimedean spiral, each of the spirals being distributed on the encoder by successive rotation of an angle
      π    Npp    .
The advantage of this type of encoder is that the polar width Lp of each of the poles along the radius of the encoder becomes independent of the number Npp of pairs of poles, thus being able to reconcile a small number of poles with an adequate positioning of the sensitive elements relative to the sinusoidality and the amplitude of the magnetic signal to be detected.
However, the prior art proposes a positioning of the sensitive elements along the radius of such an encoder, which poses a certain number of problems.
In particular, to satisfy the compromise between sinusoidality and amplitude, the sensitive elements are arranged at an air gap distance from the magnetic track which is of the order of
      Lp    2    .Thus, in particular to avoid risking a mechanical interaction between the fixed sensor and the rotary encoder, the polar width Lp must typically be between 2 and 6 mm.
Yet, in order to avoid the edge effects of the magnetic field delivered by the encoder, the sensitive elements must be positioned in relation to the magnetic track with at least one pair of poles on each side in the radial direction, i.e. two Lp on each side in addition to the radial space necessary for the arrangement of the sensitive elements.
As a result, the encoder must have a significant height, in particular greater than 6·Lp, height that may not be available in some integrations.
Moreover, the magnetic field generated by a spiral encoder on a pair of magnetic poles is the combination of a fundamental by definition perfectly sinusoidal and a plurality of odd-order harmonics, including the 3rd order harmonic that typically represents 5% of the fundamental. According to the position of the sensor and the reading air gap, this proportion of the 3rd order harmonic may be much greater.
In order to obtain an accurate determination of the rotation parameter, it is desired to measure the filtered signal of at least the 3rd order harmonic. However, any fixed compensation of the error provided by the harmonics is difficult to produce, in particular in that it depends on the measurement conditions (air gap, position of the sensor). Moreover, a calibration is also difficult to envisage for a large volume and low-cost application.
The invention aims to solve the problems of the prior art by proposing in particular a system for determining at least one rotation parameter of a rotary member, wherein the accuracy of the determination is improved. Furthermore, the system according to the invention provides a compromise between the periodicity and the amplitude of the magnetic signal detected without inducing specific space constraints for the encoder delivering the signal, and in particular in relation to a magnetic encoder with a low number of pairs of poles.
To this end, the invention proposes a system for determining at least one rotation parameter of a rotary member, the system comprising:                an encoder intended to be combined in rotation with the rotary member so as to move jointly therewith, the encoder comprising a body whereon is formed a magnetic track that is able to emit a periodic magnetic signal representative of the rotation of the encoder, the track having an alternation of North and South magnetic poles separated by i transitions, each of the transitions extending along an Archimedean spiral defined in polar coordinates in relation to the axis of rotation by the equation        
      ρ    =                            Npp          ·          Lp                π            ·              (                  θ          +                      θ            i                          )              ,Npp being the number of pairs of poles of the magnetic track and Lp the polar width of each of the poles according to the radius of the encoder, the angle θi of rotation of the ith spiral in relation to the first spiral being equal to
      π    Npp    ·  iwith i between 0 and 2·Npp−1;                a rotation sensor able to detect the periodic magnetic field emitted by the encoder by means of a plurality of magnetic sensitive elements, distributed angularly along the magnetic track to each deliver a signal representative of the rotation of the encoder, the sensor further comprising a device for subtracting the signals delivered by two sensitive elements forming therebetween an angle γ that is such that:                    0.55π<γ·Npp<0.83π, modulo 2π; or            1.17π<γ·Npp<1.45π, modulo 2π.                        